As is common, voltage regulators are employed in the charging of batteries utilized or recreational vehicles which regulators are necessary to prevent overcharging of the battery.
In recreational vehicles such as ski mobiles, motorcycles or outboard motorboats, alternators are employed with engines that have a very wide speed range. As a result the voltages from the alternator may range in the hundreds of volts. For instance, with respect to ski mobiles and motorcycles, the engines may occasionally be revved to 10,000 rpm which can result in a maximum voltage of approximately 500 volts. In wide speed range engines shunt regulators are sometimes used, in which the alternator output terminals are shorted when the battery voltage exceeds a predetermined threshold. Another approach is to open circuit the alternator when the battery voltage rises above a predetermined threshold. These type circuits utilize a drive or control transistor to render non-conductive the silicon controlled rectifiers (SCRs) used to disconnect the alternator from the battery when the battery voltage exceeds the predetermined threshold. When the silicon controlled rectifiers are rendered non-conductive, the result is extremely high open-circuit voltages being applied across the control or drive transistor. When the battery voltage drops to a point where charging is to begin, the control transistor is to be turned ON. However, if the control transistor is turned ON with a high voltage across it, forward bias second breakdown may occur and the transistor will be destroyed. In the second breakdown, the transistor first becomes conductive but is then unable to withstand the high energy surge through it.
In summary, selected drive transistors can withstand normal collector-to-emitter voltages on the order of 500 volts and can supply sufficient current to drive the silicon controlled rectifiers. However these same drive transistors are prone to second breakdown when, after the transistor is turned OFF, the transistor is to be turned ON and the collector-to-emitter voltage is high (e.g. more than 50 volts) due to a high alternator output which is the result of interrupting the connection between the alternator and the battery. This second breakdown problem may be solved by utilizing even higher power control transistors. However the cost of designing and providing such a high power switching transistor is prohibitive, i.e. unattractive, uneconomical or expensive.
By way of background, one type series shunt regulator is illustrated in U.S. Pat. No. 3,857,082 issued to L.J.K. van Opijnen Dec. 24, 1974 and is assigned to the Assignee of the subject application. In this patent a series regulator is shown in which a diode bridge utilizing SCRs is utilized to open the circuit between the alternator and the battery terminals when the battery is fully charged. A shorting circuit is also described which shorts the alternator output terminals together every time the bridge portion of the circuit opens up. This means that when the battery is full, the diode bridge is opened up and the terminals of the alternator are shorted together. While in many applications shorting of the alternator terminals does not damage the alternator, in marginally designed alternators damage through overheating can occur.
If in an effort to spare the marginally designed alternator, the alternator terminals are not shorted, the output of the alternator will rise above the forward bias second breakdown voltage for the control transistor.